This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 199 00 176.6, filed on Jan. 7, 1999 the entire disclosure of which is incorporated herein by reference.
The invention relates to a conveyor system for flowable bulk material that is supplied to the conveyor system through a specially formed funnel.
Conveyor systems for transporting flowable bulk material are known in the art. Such systems include one or two screw conveyors rotatably mounted in a housing. The flowable bulk material, for example powders, is supplied into the conveyor housing through a funnel also known as a hopper. The funnel or hopper normally has a circular funnel inlet and an orthogonal funnel outlet leading into a housing inlet of the conveyor system. Between the funnel inlet and the funnel outlet the funnel walls are formed partially cylindrical and partially as a pyramid frustum. Additionally, the funnel wall includes crowned or cambered intermediate wall sections. The side walls of the conveyor housing are positioned partly vertically and partly slanted to form a wedge shape as a trough in which one or more conveyor screws or worms are rotatably housed.
Such hoppers or filler funnels are also described in a published Dissertation entitled: xe2x80x9cOn the Characteristics of Volumetric Screw Dosing Devices for Bulk Materialsxe2x80x9d, by Dr. Dieter Fritsch, 1988, published G. Graebner, Dissertation and Offset Printing, Altendorff, near Bamberg, Germany.
The production of conventional funnels requires a substantial effort and expense. Such disadvantages have been conventionally acceptable because the free flowability of the bulk material must be assured even for bulk material that is not necessarily freely flowable and for bulk material having a tendency to build bridges in the funnel thereby causing clogging the flow through the funnel. Certain conventional conveyor systems of this type are even equipped with a stirring mechanism in the lower portion of the funnel or just above the screw conveyor in order to maintain the flowing of the bulk material.
In view of the foregoing it is the aim of the invention to achieve the following objects singly or in combination:
to construct and orient the funnel walls in such a way that an improved flowability of the bulk material is assured even for bulk material having a hard flowing characteristic;
to minimize the need for a stirring mechanism in the funnel or in the conveyor system while still maintaining an optimal flowability of the bulk material; and
to shape and configure the funnel walls in such a way that the funnel manufacture is facilitated, thereby reducing the costs for these funnels or hoppers.
The above objects have been achieved according to the invention by a conveyor system for flowable bulk material, which system is characterized by a combination of the following features. The present system comprises a conveyor that is mounted in a conveyor housing provided with an upwardly facing housing inlet having four inlet edges forming together four housing inlet corners. A funnel for feeding bulk material into the conveyor housing through the housing inlet is provided with a circular funnel inlet and a funnel outlet fitting or matching the housing inlet.
The funnel walls are formed by a cylindrical wall section that surrounds the circular funnel inlet and by a total of eight wall sections, four of which form lateral plane or flat wall sections arranged in first and second pairs of lateral plane wall sections, whereby the wall sections forming a pair are positioned diametrically opposite each other. The upper ends of the plane wall sections are connected to the cylindrical wall section while the lower ends form the funnel outlet. The upper ends of the lateral wall sections are narrower than the lower ends of the lateral wall sections. The eight funnel walls further include four curved corner wall sections having a wider upper portion than a narrower lower portion so that wider and narrower wall sections alternate with each other around the funnel inlet and around the funnel outlet. The four curved corner wall sections are curved by reason of a curved cross-section in a horizontal plane. The cylindrical wall section has an axial width that is substantially smaller than an axial length of the flat or plane lateral wall sections and also substantially smaller than a longitudinal dimension of the corner wall section. The longitudinal dimension of the curved corner wall sections is approximately equal to the axial length of the lateral plane wall sections.
The meaning of the terms xe2x80x9capproximatelyxe2x80x9d and xe2x80x9csubstantiallyxe2x80x9d as used herein is determined by the steepness angles xcex1 and xcex2. The angle xcex1 is defined as the angle between a vertical line and a curved corner wall section. The angle xcex2 is defined as the angle between the vertical and a lateral plane wall section. When xcex1 and xcex2 are equal to each other and the funnel outlet is a square rather than a rectangle, the axial length of the lateral walls and the longitudinal dimension of the corner wall sections will be equal to each other. However, according to the invention it is preferred that the angle xcex1 is within the range of 0xc2x0 to 5xc2x0 and the angle xcex2 is within the range of larger than 5xc2x0 up to 15xc2x0. It has been found, that this arrangement in combination with the relative length between the curved corner wall sections and lateral plane wall sections on the one hand and the vertical width of the cylindrical wall section provides steep positions of the lateral and corner wall sections relative to the vertical to thereby positively avoid clogging of the bulk material, for example by so-called bridging of the bulk material flowing through the funnel.
The present invention is also directed to the funnel itself independently of the conveyor system. Such funnel is defined by the above outlined funnel features.
Relative to the entire axial length of the funnel according to the invention its lateral flat or plane wall sections and its curved corner wall sections extend at a slant relative to the funnel outlet which slant is substantially steeper than in known funnels. It has been discovered, that this steepness is critical to the prevention of clogging, for example by bridging of the bulk material flowing through the funnel. This steepness of the lateral wall sections and of the corner wall sections make it surprisingly possible to construct the axial length of the funnels according to the invention substantially shorter than was possible heretofore, thereby, saving sheet material for making the present funnels. Thus, the lateral flat or plane wall sections which resemble a cross-section through a pyramid frustum and the corner wall sections which have a curved cross-section in a horizontal plane and which are approximately triangular in a developed projected form, provide together with the short cylindrical inlet wall section an integrated unit which provides optimal flow characteristics for the bulk material. A shorter funnel according to the invention achieves a throughput that can be achieved by a conventional funnel only if the conventional funnel has an axial length larger than the axial length of the present funnels. Comparing a funnel according to the invention with a conventional funnel of equal size, namely of equal axial length, equal inlet cross-sectional area, and equal outlet cross-sectional area, the present lateral plane wall sections are about twice as long as conventional lateral wall sections, which is possible because the inlet cylindrical wall section of the present funnels is very short compared to the length of the lateral and corner wall sections of the present funnels. Similarly, the corner wall section having a cross-sectional curvature in a horizontal plane are several times longer and thus steeper than conventional cambered or crowned corner wall sections. The present wall sections which, due to gravity, actively guide the bulk material, are distinctly steeper than comparable wall sections of conventional funnels. As a result, the present funnels offer a substantially improved flow characteristic to the bulk material, while still assuring the required guiding of the bulk material into the conveyor housing.
The several dimensions discussed herein vary in accordance with the required cross-sectional areas of the funnel inlet and of the funnel outlet. The dimensions of the funnel outlet will depend on the housing construction and may be either rectangular or square. The cylindrical wall section of the present funnel inlet may include a flange extending radially or it may be provided with a conical collar that in turn is provided with a flange.
Although it is the purpose of the invention to generally avoid the use of a stirring mechanism, this does not exclude the use of such a stirring mechanism, for example, in connection with a bulk material having especially difficult flow characteristics.